<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    .box {
      width: 300px;
      height: 300px;
      background-color: #bfa;
      overflow: auto;
      position: relative;
    }
    .box>div{
      height: 100%;
      width: 300px;
      border: 1px solid;
    }
    .box3{
      background-color: #fff;
      position: absolute;
      top: 400px;
      left: 0;
    }
  </style>
</head>
<body>
<!--相对定位的top值 在有滚动条的情况下 是相对父盒子   总高度计算的-->
 <div class="box">
   <div>1</div>
   <div>2</div>
   <div class="box3">3</div>
 </div>
</body>
</html>
